Automatic frequency control apparatus



. Nov. 27, 1945. L. F. MAYLE 2,389,992

I AUTOMATIC FREQUENCY CONTROL APPARATUS I Filed April 15, 1943 INVENTOR ATTORNEY Patented Nov. 27, 1945 AUTOMATIC FanounNcifcoNrsoL I APPARATUS Louis F. Marie, Fort Wayne, Ind., asslgnor to 'Farnsworth Television and Radio Corporation, v a corporation of Delaware Application Apr-n15, 1943, Serial No. 483,138

1 Claim.

This invention relates to automatic frequency control apparatus and particularly to such apparatus used in conjunction with the relatively high frequency oscillator of a television timer.

According to conventional television practice it is customary to employ timer apparatus for definitely correlating the frequency of the synchronizing signals with a source of standardizing frequency such as a commercial alternating current power supply. The timer apparatus includes a high audio frequency oscillator and a timer chain or frequency divider by means of which the oscillations of twice horizontal scanning frequency (where interlaced scanning is employed) are converted into control impulses at the frequency of the standardizing source.

In order to control the frequency of the oscillation generator so that the frequency of the oscillations is related at all times to a standardizing frequency there also are derived from the frequency divider relatively low frequency impulses which are compared with impulses of substantially the same frequency derived from the standardizing source. Any deviation of the frequency of these impulses is detected and a control voltage is generated representative of the deviation. The control voltage is utilized to adjust the frequency of the oscillator to restore it to the desired relationship with the standardizing frequency.

A conventional form of oscillator generally isn for the control of the usual type of frequency divider for the reason that it does not possess a wave form which is sufficiently steep to operate the frequency divider in the most accurate man ner. An oscillator of the multivibrator type is better suited for this purpose since the voltage impulse generated thereby is one which has a very steep leading edge. In one of the simpler forms of multivibrator, the frequencycontrol circuit is of relatively low impedance. Usually the impedance of the circuit in which the frequency control voltage is developed is relatively high.

object of the present invention therefore, is to provide a meansfor impressing a frequency cmtrol voltage derived from a relatively high impedance circuit upon a relatively low impedance frequency-determining circuit of a multi vibrator type of oscillator.

In accordance with the present invention, there is provided an arrangement of electronic devices for generating periodic impulses having steep wave fronts and in which the impulsive voltage is derived from a relatively low impedance device connected in the cathode circuits or the electronic devices. The periodicity of the impulses ,is relatively high. These impulses are applied to a chain of electronic devices commonly known as a timer chain or frequency divider whereby substantially any sub-multiple frequency of the generated frequency may be derived. One of these sub-multiple frequencies corresponds to the frequency of the commercial source of alternating current power which is converted to supply direct current energy to the television transmitting apparatus. A series of impulses is derived from the alternating current power supply at the frequency of the supp These impulses are compared by a frequency comparator with similar impulses of substantially the same frequency derived from the frequency divider. A

difference in frequency produces a control voltage. The polarity of this voltage is determined by the sense of the deviation of the oscillator derived impulses with respect to the power supply impulses, and the magnitude of the control voltage is determined by the amount of the frequency deviation.

This control voltage is developed in a circuit of relatively high impedance and is impressed upon the input circuit of an electronic device having a relatively low impedance device connected in the cathode circuit thereof. The voltage developed in the cathode circuit impedance device corresponds to the voltage applied to the control grid of the tube. The cathode impedance potential is impressed upon the low impedance frequency control circuit of the high frequency impulse generator, whereby to alter the frequency thereof in sense and magnitude corresponding to the product of the frequency comparison.

For a better understanding of the invention together with other and further objects thereof, reference is bad to the following description taken in connection .with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, the single figure is a circuit diagram of a high audio frequency generator embodying the instant invention.

Having reference to the drawing, there is shown a high frequency impulse generator of the multiwmwpewmnnmmneema aemama. ambntinmnfilcettheynmaybembmed inamlcenuelcpetuhemiihetmunniodetvm.

mmcincentpommvmtheap mams is obtainedffromtenmimlslmd. mommassume spaoecumntofthe'mbe I intheeacnodereslsmanner, anemone omen-t from 1:. commercial power ".Ilhe control grid of the wee Q is connected holfihe junction or two series mesistorsfi and [6,, munecbed to the i lib'ive negaifiiue dime; (commit te minals 3 and respectively. The mlhudcs of both tubes l and 72 are connected no tlhesnegaitiue direct current oer-- lmlnal l lthmough so mammary small :neslstor TI. The anode of the tube (I (commuted fnhnough a resistor 28 to posililue direct eurcent terminal 3,

tnrlLtherebydrlvingthecathodeofmbeIlmo greater positive potential with respect to the congrldoi thlstube. Slnoethepolentiall of the control grid of tube I with respect to ground is unchanged, the effect is an increase ln the negafive grld-to-catbode potential of this tube, thereby initiating adecrease in the conductance tweoi.

:a consequence of the decreased conductance of 'the tube l, the drop across the resistor 8 decreases correspondingly to cause the potenrial o! the anode of the tube to increase positive- The anode (or tube 2| coupled to the control {grid of tube 2 by :a condenser The Junction polntbetween the condenser and the control grid of the babe 52 is connected 'l'hrough an adjustable leak resistor (or potentiometer Ill and a relatively small resistor lit to the negative direct current The anode of the tube '2 connected (directly to the positive current terminal.

The voltages-(developed resistor are al plied to a ireouency 1|?! which under confirm] or these voltages to produce voltages ffnequencies which .are sub-multimes of the itmequency (of voltage developed in theresisl/or "I. The frequency corresponding to the frequency or the alternatin current power is applied to a frequency comparator I3, Ethane applied to the comparator, derived from the alternatin current power'supply. The voltage which is the result or the comparison by the irequency comparator (I3 is impressed upon the input circult of :a :trequmcy control hube El-4.. The input circuit of this tube includes control grid and cathode electrodes thereof the resistor II. The anode of the W116 M is ected directly in the positive current terminal so that th output load includes, externally of the tube, only the resistor 2H. tube thus functionsasacathode Iollowem.

Referring now to the operation of the apparatus embodying instant invention, attention will be directed first to the functioning of the high frequency generator without referring to the frequency control apparatus. Assume that the condenser 8 is charged to a potential appreaching the current potential of the termlnals '3 and By reason of -the condenser charge, the control grid of the tube 2 is highly negative with respect to its associated cathode. Space current in this tube, therefore, is .cut 05. At time assume also that the tube I is rendered conducting. Then the anode of this tube is at a less positive potentialthan that of the lyl The increased positive anode potential is applied to the grid of tube 2 by means of the con denser 9, whereby to increase still further the conduction of space current in this Thus,

the volt-age drop across the resistor Tl also is inpotential eflects a relatively large change in the Consequently, the in potential of the-anode. crease in the positive potential of the anode of tube l is much greater than the increase in the positive potential of the cathodes of tubes I and 2, with the result that the grid voltage of tube 2 with respect to ground increases at a. much greater rate than the cathod voltage of this tube with respect to ground. As a. result, the interaction be tween tubes l and 2 is strongly regenerative, In operation, the space current in tube I is interr pted abrup ly and space current in tube 2 is in? itiated abruptly.

With tube I non-conducting, the potential of the gridof tube 2 tends to become positive with respect to its associated cathode. The grid and cathode elements then function as a. diode and charging current for the condenser 9 flows through resistor 8 to the cathode of tube 2. As the condenser approaches its fully charged state, there is an exponential decrease in the chargin current which efi'ects a decrease in the positive grid-to-cathode voltage of tube 2. Thus, there is produced a corresponding decrease in the conductivity of the tube which decreases the current flow through the resistor I. drop across this resistor decreases as a result of the decreased current flow, thereby increasing the grld-to-cathode voltage of the tube I gradually, untll the point is reached where this tube is rendered conducting.

cathode of tube 2 is made less negative. When the condenser discharge has progressed to the point where the grid-to-cathode potential of the tube 2 is at the point olf anode-to-cathode current cutofL the tube 2 is on the threshold between its noneonducting and its conducting state. A further incremental discharge of the condenser raises the grid potential of tube 2 sufliciently to Immediately upon the iniflatlon of the con duction of space current-in the tube I, there is produced a voltage drop across the resistor 8 whereby the anode voltage 0! the tube is renof the specified amplification factor of the tube I the interaction betweenthe tubes is strongly regenerative. Consequently, space current In the tube 2 is interrupted abruptly and space current in the tube I is initiated abruptly.

Because of thefact that the output circuit of tube I includes the resistor 8 while the output. circuit of tube 2 is not provided with any appreciable corresponding impedance, the current flow mane the condom of me: count in this The voltage through the resistor I is much greater when tube 2 i conducting than it is when tube 1 is conducting. Consequently, the voltage develope across the resistor I is subject to a corresponding variation. Thus, this voltage is a series of impulses having very steep leading and trailing edges. The crest of each of these impulses decreases'exponentially from the leading to the trailing edges as a result of the described decrease of the charging current for condenser 8 occurring during conductive periods of tube 2.

Immediately upon the initiation of the conduction of space current in tube I, there is evidenced a large decrease in the voltage applied to the cathode of tube 2. The result of this voltage decrease is to drive the grid-,to-cathode voltage of this tube far beyond the point of space current cutoff. This condition is realizable by the functioning of the condenser 9 which prevents the grid voltage from changing suddenly.

However, the condenser charge is gradually dissipated through the potentiometer I0, thereby to allow the voltage of the grid of tube 2 to approach gradually its space current cutoff value. When this potential is reached, the conduction of space current in tube 2 again is initiated and V the cycle is repeated as described.

The frequency of the impulsive voltage developed in the cathode resistor 1 is a function of the grid bias of the tube 2. This bias is positive with respect to ground and the magnitude thereof controls the operating time of the tubes. Thus, to increase the frequency of the impulsive voltage the bias is made more positive and to decrease the frequency of the impulsive voltag the grid bias is made less positive.

This impulsive high frequency voltage is applied to the frequency divider from which is obtained an impulsive voltage of relatively low frequency. If the frequency of the alternating current power supply is, say 60 cycles, thenthe frequency of the relatively low frequency impulsive voltage derived from the frequency divider I2 is also substantially 60 cycles.

When the frequency of the impulsive voltage derived from the frequency divider I2 is exactly equalto the frequency of the alternating current power supply. there is developed by the frequency comparator ii a. control voltage for the input circuit of the tube H of a magnitude such that there is produced a current flow throughthe tube and the cathode follower resistor ii suillcient to produce a voltage drop in the resistor ll of a normal magnitude. The voltage developed in the resistor II is impressed by means of the potentiometer it upon the control grid of the tube 2 so as to produce the normal positive bias of the grid with respect to ground.

rived from the frequency divider is greater than the frequency of the alternating current power supply, the frequency comparator produces a voltage to decrease the current in the tube It and in the resistor ll whereby to decrease the voltage developed in this resistor. This decreased voltage, when impressed upon the control grid of the tube 2, increases the positive bias thereof willciently to decrease the frequency of the generated impulses to compensate for the deviation.

While it will be understood that the circuit specifications of the controlled frequency oscillator embodying the invention may vary according to the design for any particular application, the following circuit specifications for a generator employed for the use described, are included by way of example only, as follows:

Tubes I and Ln 6SL7GT Voltage between terminals I and 4 vo1ts 150 Resistor 5 ohms 100,000 Resistor 6 do 10,000 Resistor do 3,000 Resistor 8 "do--- 50,000 Condenser 9 micro-microfarads 200 Potentiometer l0 ohms 50,000 Resistor ll do- 1,800 Tube l4- 6J5GT While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and therefore, it is aimed in the appended claim to cover all such changes and modiflcations as fall within the true spirit and scope of the invention. 1

What is claimed is:

-In a television timer, a multivibrator having two electron discharge devices connected for selfsustaining alternate operation at a relatively high frequency determined by the input circuit biasing voltage of one of said devices, means for derivin from said multivibrator a voltage of substantially a predetermined frequency, a source of standardizing frequency substantially equal to said predetion in said frequencies, a circuit including an When the frequency of the voltage derived from the frequency divider is less than the frequency of the alternating current power supply, there is produced by the frequency comparator, a voltage which, when applied to the input circuit of the impedance device for developing a biasing voltage for said input circuit representative of said detected deviation, whereby to adjust the frequency of said multivibrator so as to be definitely related to said standardizing frequency. and a vacuum 

